There are at least two conventional multiuser superposition transmission schedules. One is Non-Orthogonal Multiple Access (NOMA), and another is Rate-Adaptive Constellation Expansion Multiple Access (RA-CEMA/REMA).
Non-orthogonal transmission technology has been discussed extensively due to its better cell coverage and higher throughput for users located at cell edge region than the traditional orthogonal multiple access (OMA) technology. In order to improve user throughput of OMA, user equipments (UEs) of non-orthogonal transmission technology would need to enhance their receivers with interference cancellation capability in order to eliminate interferences generated by other users.
FIG. 1 illustrates a conventional NOMA transmitter. The NOMA transmitter is assumed to transmit user data of a near UE (UEN information 101) and user data of a far UE (UEF information 111). The UEN information 101 would be encoded by a Turbo Encoder 102 to generate encoded UEN information 101 (CN) which would then be modulated by a Pulse Amplitude Modulator (PAM) 103. Similarly, the UEF information 111 would be encoded by a Turbo Encoder 112 to generate encoded UEN information 111 (CF) which would also be modulated by a Phase Amplitude Modulator (PAM) 113. The output of the PAM modulator 103, SN, would be power scaled by the square root of PN by a multiplier 104, and where the output of the PAM modulator 113, SF, would be power scaled by the square root of PF by a multiplier 114, where (PF>PN), and PN+PN=1. After the output of the power scaling would be summed by an adder 121 to generate a transmitted symbol Z, where Z=√{square root over (PN)}SN+√{square root over (PF)}SF. Notice that for the NOMA transmitter, the UEN information 101 and the UEF information 111 to be transmitted would have a different constellation diagram. For the example of FIG. 1, the UEN information 101 to be transmitted could be modulated according to 2-PAM whereas the UEF information 111 to be transmitted could be modulated according to 4-PAM. The two constellation diagrams which are originally Gray Mapping after combining will become Non-Gray Mapping. The joint constellation would increase the complexity of a wireless demodulator and decoder in the receiving end.
FIG. 2 illustrates a conventional REMA (or RA-CEMA) transmitter. For the REMA transmitter, user data of a near UE (UEN information 101) to be transmitted would be encoded by a Turbo Encoder 102 to generate encoded UEN information 101 (CN), and the user data of a far UE (UEF information 111) to be transmitted would be encoded by a Turbo Encoder 112 to generate encoded UEN information 111 (CF). Instead of being modulated right away, both CN and CF would be transmitted a REMA function 221. The REMA function 221 essentially takes both the both CN and CF and operates them by a transmission matrix. The transmission matrix as shown in FIG. 2 would include rows of near UE 231, rows of far UE 232, and columns of available REs 233. The CN and CF would be operated by the transmission matrix and undergo row permutation to generate an output of the REMA function (I). The output, I, could be modulated by a conventional modulator such as a PAM modulator 222 and thus would require a less complicated receiver relative to the design of FIG. 1.
FIG. 3A illustrates a MUST category 1 (NOMA) transmitter. The MUST category 1 transmitter is similar to the design of FIG. 1 as two UE information (TB1, TB2) to be transmitted are sent to encoders (302a, 302b) and subsequently modulators (303a, 303b). The modulated UE information are allocated with different powers by a power allocation block 304 and subsequently summed by an adder 305. The detail with regard to the MUST category 1 transmitter is included in 3GPP TR 36.859 which is incorporated by reference for all purposes.
FIG. 3B illustrates a MUST category 2 (NOMA with Gray mapping) transmitter. The MUST category 2 transmitter is similar to the design of FIG. 1 & FIG. 3A as two UE information (TB3, TB4) to be transmitted are sent to encoders (312a, 312b). The encoded UE information is transmitted to a joint modulation and power allocation block 313 are allocated with different powers by a power allocation block 313 and subsequently summed by an adder 314. The joint modulation and power allocation block 313 are a combination of modulators 303a 303b and power allocation block 304 but with Gray mapping. The detail with regard to the MUST category 2 transmitter is included in 3GPP TR 36.859 which is incorporated by reference for all purposes. The downlink MUST transmitter for LTE (Rel-14 WI) and its joint constellation is shown in FIG. 4.
FIG. 5 illustrates a MUST category 3 (REMA) transmitter. The MUST category 3 transmitter is similar to the design of FIG. 2 as two UE information (TB5, TB6) to be transmitted are encoded to encoders (502a, 502b) and subsequently transmitted to a block 503 which is similar to the transmission matrix of REMA function block 221 of FIG. 2. The output would subsequently be transmitted to a legacy modulation mapper 504. The detail with regard to the MUST category 3 transmitter is included in 3GPP TR 36.859 which is incorporated by reference for all purposes.
In current LTE standard, user data would be individually turbo-encoded and then multiplexed before high-order modulation is performed. FIG. 6 illustrates a conventional MUST transmitter according to the current LTE standard. The MUST transmitter of FIG. 6 would take information bits of a UE and encode the information bits of the single UE by one turbo encoder 602 with an encoding rate R. The turbo encoder 602 would generate a binary string of L binary levels which are sent to a PAM modulator 603 in parallel. The input of the PAM modulator 603 would thus be a transmission symbol with a plurality of binary levels (e.g.) Xlevel0, Xlevel1, Xlevel2, XlevelL-2, and XlevelL-1) as shown in FIG. 6.
FIG. 7 illustrates a conventional non orthogonal MUST transmitter by combining information of two UEs. In non-orthogonal transmission technology, the informational signals that are transmitted from the BS respectively to the two UEs are independently generated. In FIG. 7, the information bits of near UE, bN, is sent to a turbo encoder 702a with a code rate RN, and the information bits of far UE, bF, is sent to a turbo encoder 702b with a code rate RF. The output of the turbo encoders 702a 702b would be combined by symbol-level/codeword-level MUST scheme.
It is worth noting that typically a single user information would be encoded by a single encoder with a specific code rate. Also, currently all the levels in a transmission symbol have the same code rate. However, allocating the same transmission rate to all the levels in a transmission symbol would make not be optimal for the overall performance of a system.